Grape

Anthracnose (fungus – Elsinoe ampelina): This disease is often called “birds-eye-rot” because of the circular, sunken, ashy gray, dark-bordered spots on the berries. It attacks fruit stems, leaf veins, petioles, tendrils, and young shoots in addition to the berries. The berry often cracks to the extent of exposing the seed. The disease overwinters in old lesions. Periods of high humidity favor disease development. Fungicides used for black rot will help prevent this problem.

Black Rot (fungus – Guignardia bidwellii): Black rot shows up as small, reddish-brown spots on the upper leaf surface. In older lesions the margin is a black line while the inner area of the spot is brown. Small, black dots are also visible in the center of the lesion. Infected fruit shrink until they are dried mummies. The first stage of development on the fruit is small, light-colored lesions with black borders. In advanced stages the fruit is marked with the small, black dots just like the foliage. Young grape foliage is most susceptible to this disease. Disease development is favored by high temperatures and humidity. A rainy period followed by two to three days of foggy weather favors development of this disease. Preventative fungicides along with resistant varieties should be used to control black rot.

Chlorosis: Chlorosis in grapes is usually caused by iron deficiency. American varieties are particularly prone to have this problem. The leaves turn yellow, but the veins remain green. If not remedied, chlorosis will decrease the yield, reduce sugar content of the fruit, and eventually kill the vine. Two applications of iron sulfate or iron chelate during the growing season should control chlorosis. In high pH soils, iron sulfate and some iron chelate may become tied up and unavailable to the plant. Foliar applications can be made, or a chelate especially for alkaline and calcareous soils, such as Sequestrene 138Fe, can be used.

Dead Arm (fungus – Eutypa armeniacae): This fungus invades grape tissue slowly. Spring symptoms on developing shoots and leaves adjacent to an infected pruning stub may not appear for four to five years. Cankers form on vines, and nearby leaves become chlorotic and dwarfed. Cordons die later, producing the “dead arm” symptom.

Downy Mildew (fungus – Plasmopara viticola): Downy mildew is common in cool, humid environments but seldom occurs in hot, dry areas. Downy mildew is first observed as a pale yellow area on the upper surface of the leaf. The underside of the leaf is marked by a downy appearance. As the disease advances the infected tissue dies and turns brown. Young stems become thickened and are often covered with the white fruiting structures. Fruit that is infected is covered with the white growth or it turns the berry a dull green and then brown. Downy mildew is a particular problem in areas of high humidity. The disease develops in temperatures of 50oF to 60oF. Some American grape varieties show resistance, but additional chemical protection is usually needed. Most European varieties are very susceptible. Fungicide application should begin before bloom and continue at seven day intervals.

Grapevine Fanleaf (virus): Fanleaf is caused by a virus that also causes yellow mosaic and veinbanding. All three are transmitted by the nematode Xiphinema index. Leaf symptoms that resemble a fan are very conspicuous on Mission and French Colombard. Infected vines have shortened and more irregular internodes. Lateral sprout development, double nodes, and stem faciations cause a bushy appearance. Many berries shatter, and others do not develop beyond shot size. Diseased vines should be removed.

Leaf Roll (virus): Leaves roll downward and turn red between the veins progressing toward the cane tips. In California, symptoms appear in early June in non-irrigated vineyards and in August in irrigated vineyards. Vines have fewer, smaller clusters per vine with berries that are low in sugar. Red fruit varieties, such as cardinal and Mission, develop fruit lacking color, and berries of white grapes, such as Thompson seedless and Reisling, develop a yellowish-white color instead of the normal greenish-white. In general, leaf roll decreases fruit color, raises the acidity, and delays ripening. The disease is spread by propagation from infected mother vines.

Pierce’s Disease (bacterium – rickettsia-like): Early summer symptoms include delayed shoot growth, leaf mottling, and dwarfing of new shoots. Late summer and fall symptoms are burning, scalding, or drying of leaves; wilting or premature coloring of fruit; and uneven cane maturity. Ribier is very susceptible, usually dying within two years. Thompson seedless and most other French (Vinifera) varieties die within two to five years. American (Lambrusca) varieties often live longer than five years. No effective control is known. Pierce’s disease is spread by several types of leafhoppers, by the spittlebug, and by grafting. Seventy-three plant species serve as disease reservoirs and hosts for these vectors.

Powdery Mildew (fungus – Uncinula necator): This fungus grows on all above ground parts of the vine. Powdery mildew causes curling and withering of young leaves and dark staining on the surface of mature leaves. It may appear as a gray powdery growth on canes, and when rubbed off, it leaves web-like, dark-brown discolorations. Other symptoms include dropping, discoloration or splitting of berries, browning, and poor maturation of canes. American grape varieties are rarely damaged by this disease. Since the fungus is favored by low humidity and can grow at 90oF, this disease is more common in West Texas. Prevention is the best means to control powdery mildew. It should be controlled by fungicides applied when foliage first develops and repeated at two to three week intervals until berries are full size. If newer fungicides are not available, sulfur dust applied at the rate of 5 to 10 pounds of dusting sulfur per acre will prevent powdery mildew from developing. Apply dust when shoots average 6, 12, and 18 inches in length. Make additional applications every two weeks until fruit matures. Reapply sulfur after rains. If early sprays are omitted, heavy infections can be controlled with wettable sulfur at 1.5 pounds/100 gallons with a wetting agent. Do not spray table grapes with this mixture if berries are more than one-third full size. Sulfur dust or spray can burn leaves, shoots, and fruit when the temperature is over 90oF; no applications should be made at such times.

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